The authors investigated the risk of developing dementia for persons aged 50-75 years who suffered from Parkinson's disease, epilepsy, or severe head trauma. They compared the risk in this patient group with the risk in a reference group in a follow-up study based on the linked databases of three Dutch nationwide morbidity registers over the years 1980-1989. The overall relative risk of developing dementia within 8 years in patients with Parkinson's disease who were initially free of dementia was 3.0 (95% confidence interval (CI) 2.9-3.1). Risk was especially increased in younger Parkinson's disease patients (relative risk (RR) = 13.2, 95% CI 6.2-28.6). For patients with epilepsy, the overall relative risk was 1.5 (95% CI 1.4-1.7). Severe head trauma was not associated with an increased risk of dementia (RR = 1.0, 95% CI 0.9-1.1). These findings suggest that Parkinson's disease is an important risk factor for dementia, with a particularly high risk in young patients with Parkinson's disease. Patients with epilepsy may bear a moderately increased risk of developing dementia. This study does not support earlier findings in retrospective case-control studies of an increased risk of dementia in head trauma patients.